Grid pathfinder and collision detection with OnTriggerEnter

Hi, I've made a grid pathfinder and need to correctly setup whether a cell is blocked or not by a collider.

Bounding boxes are not accurate enough for my purpose since I have some meshes with "holes" that must not block all the cells they are on.

Raycasting seems not to be a good solution either because by design they return false if casting from inside a collider (hence failing to detect cells that are fully inside a collider).

OnTriggerEnter seems to be my best chance, for instance by creating mini-colliders for each of my cells, but because of huge grids (~20,000 cells) I need to find some performance workaround. For instance, I could create mini-colliders for few seconds to detect static meshes, then handle dynamic objects differently. So my question is how to have more control on OnTriggerEnter ? What's the optimal amount of time I should wait for OnTriggerEnter to be called (or not) to declare a "static" cell free ? Only one FixedUpdate frame or more ? Any insight on my issue (or maybe another solution) would be highly appreciated :)

Edit: Ok I've actually tested and it seems I can declare free a cell when entering the second FixedUpdate frame if OnTriggerEnter was not called before. I'll assume this hypothesis is correct and will build my system around it, hoping it is consistent.